The lac operon is a set of genes in E. coli bacteria that helps them digest lactose, a sugar found in milk. It consists of three main genes: lacZ, lacY, and lacA, which code for proteins that break down lactose into simpler sugars. The operon is regulated by a promoter and an operator, allowing the bacteria to turn the genes on or off depending on the presence of lactose. When lactose is available, it binds to a repressor protein, causing it to detach from the operator. This allows RNA polymerase to access the promoter and initiate transcription of the lac operon genes. In the absence of lactose, the repressor remains bound, preventing gene expression and conserving energy.